The present application is a US national phase application of PCT/IL97/00217, filed Jun. 29, 1997.
This application is related to precise distortion and error correction in Gamma Camera systems and in particular to incremental calibration thereof.
FIG. 9 is a schematic illustration of an Anger camera. A subject 110 ingests, or is injected with, a radio-pharmaceutical, which tends to concentrate in certain body tissues, such as tissue 112. The local concentration depends on the particular radio-pharmaceutical, the tissue type and on its metabolic processes. Periodically, the radio-pharmaceutical generates a gamma photon. Although gamma photons are emitted in all directions, some of them travel along a straight path 114 and through a collimator 115 so that they might interact with a scintillator crystal 116. The interaction between the gamma photons and crystal 116 generates a shower of photons 118 in the visual light range. The number of secondary photons is directly dependent on the energy of the original gamma photon.
The number of photons 118 can be estimated by detecting these photons with a plurality of photo-multiplier tubes (PMT), such as tubes P1 to P4. The energy of the gamma event is then calculated by a position and energy calculator 126 which sums the contributions of all the individual PMTs. If the photon is scattered along its path from tissue 112 or if it is a cosmic ray photon, its energy will not be the same as the energy of a gamma photon as emitted by the radio-pharmaceutical. Thus, the number of photons 118 will also be different from those in xe2x80x9cnormalxe2x80x9d interactions. By windowing the detected events, so that only events with an energy within a desired range are taken into account, events which do not form a portion of the image may be rejected.
A collimator 115, which is usually a fan collimator or a parallel-hole collimator is used to project the distribution of the radio-pharmaceutical in subject 110 onto detector crystal 116. The position of the interaction of the gamma photon on (or in) crystal 116 indicates the travel path 114, since collimator 115 limits the possible paths of a gamma photon from subject 110. This position is calculated by calculator 126. An accumulator 128 accumulates calculated interaction locations and builds an image therefrom, which is displayed on a display 130.
There are several known methods of calculating the interaction position, the most commonly used having been invented by Anger. In the Anger method, the determined interaction position is a weighted average of the positions of the PMTs which detect the interaction. the weighting being the number of photons detected by each PMT. There are several problems with this method. First, the sensitivity of the PMTs are not the same. Thus, a calculated position will tend to be displaced towards the position of the most sensitive PMT. Second, the sensitivity of PMTs changes with time, especially when an old PMT is replaced with a new one. Third, PMTs tend to be more sensitive at some angles than at others. Fourth, at the edges of crystal 116, some photons are lost, either by escaping the crystal or by there not being sufficient PMTs surrounding the interaction position from all directions. Fifth, different regions of crystal 116 differ in their sensitivity to gamma radiation and produce different amounts of light from an interaction of the same energy. Sixth, some portions of the crystal interact more strongly with the gamma radiation and thus generate a higher number of events for a fixed amount of radiation. Moreover, not only are position calculations inaccurate; as a second result of these problems, so are energy calculations. Seventh, the amount of light reaching the PMT is not linearly related to the distance of between the event and the PMT.
The results of these problems are generally classified as linearity errorsxe2x80x94position determination is not exact; energy errorsxe2x80x94energy determination is not exact; and sensitivity errorsxe2x80x94the count of interactions at crystal 116 is not in a fixed proportion to the number of impinging photons.
One widely used methodology for correction of gamma cameras is the so-called xe2x80x9ctriple correction,xe2x80x9d versions of which are described in U.S. Pat. Nos. 4,424,446 and 4,588,897, the disclosures of which are incorporated herein by reference. These patents describe a correction system which corrects for geometric (dislocation) distortions, energy response variations and non-uniform sensitivity of the camera. Preferably, the sensitivity correction is performed after the other two corrections, which can be performed in any order. However, such calibration maps take a long time to prepare and must be individually created from scratch for each camera, camera-collimator combination and event energy.
In general, as disclosed in the two abovementioned patents and in other patents and publications, the camera is flooded by a source of radiation. For the determination of the energy correction, the spectra of a total signal associated with events at particular positions on the surface of the camera are acquired. An energy window corresponding to valid events is adjusted to account for the variations with position of the signal spectrum acquired. Alternatively, the signal is adjusted as a function of its position and the window remains constant. For geometric distortion correction, an image of a plate having holes at regularly spaced intervals is acquired. The measured hole positions are compared to the known spacings of the holes and a correction map, later applied to actual events during imaging, is determined. To correct for sensitivity, a flood field is applied to the gamma camera. A flood field image, which is acquired, is corrected for both distortion and energy. Remaining variations in the resulting image, which are the result of incomplete correction of the energy and dislocations errors, as well as intrinsic sensitivity variations of the camera and collimator, are used to form a normalization map which is applied to events or images after energy and dislocation corrections to correct for the sensitivity variations.
While many cameras perform triple correction on the imaging data, some cameras perform only one or two corrections.
In general, the determination of the correction maps is a fairly long and tedious process. This is caused by the fact that the data acquired in determining the corrections is based on individual events of relatively low frequency. The data generally has a substantial standard deviation of energy, position and sensitivity. Thus, in order to achieve the statistical accuracy necessary to correct the camera, a large number of events must be acquired at closely spaced positions on the camera This is especially true when large corrections must be made, in which case the number of events and the amount of time necessary to acquire them is especially large. Since, in general, camera correction maps must be periodically field updated, the camera is designed such that the amount of correction required is limited, by compromising the design of the camera. This limitation in the required correction reduces the amount of correction required and hence the time it takes to determine the correction.
A typical calibration time is about three days, For this reason, calibration is typically performed only about once a year.
The problem of calibration times is especially limiting for distortion and energy correction. It is well known that for best spatial resolution there is an optimal spacing between a scintillator plate and photodetectors used in a gamma camera. It is also well known that, at this optimal spacing, the amount of geometric distortion and the amount of energy correction required is very large. This would require determining correction at a large number of individual holes in order to assure accuracy over the entire face of the camera. Since the resolution of gamma cameras is limited, this would require acquiring a substantial number of images of relatively widely spaced holes, each image being shifted in both transverse directions to form the complete matrix required. In a highly distorted camera, the amount of data and the complexity of the correction are such that, in fact, it is not practical to correct for them. Thus, Gamma cameras are generally designed with less than optimal geometric resolution.
U.S. Pat. No. 5,293,044, describes a method for rapid localization of events using pattern matching. A pattern of PMT responses is serially compared to a stored set of PMT responses until a match is found, thereby defining the location of an event to which the PMTs responded. As can be appreciated, higher resolution of localization requires a greater number of stored PMT responses and more time to find an optimal match.
U.S. Pat. No. 5,285,072, describes an improvement of the U.S. Pat. No. 5,293,044 patent in which multiple simultaneous events are separated by matching the PMTs response to the events with generated combinations of PMT responses. The generated combinations are generated by combining at least two stored PMT responses.
It is an object of some embodiments of the present invention to provide a calibration method for a nuclear camera, in which a portion of the calibration process may be generalized for several cameras. In a preferred embodiment of the invention, recalibration of a camera after replacement of PMTs therein is a significantly shorter process than a complete calibration process.
The present invention, in one aspect thereof, seeks to provide a method for correction of Gamma cameras and other devices having similar correction requirements, in which the determination of a correction map, in the field, can be made more efficiently, for example, using fewer counts and/or with the acquisition of fewer images, so that the determination can be performed more quickly and easily, without loss of accuracy in the final image.
This method allows for designs having improved spatial resolution and accuracy and a reasonable requirement for in-field calibration.
The method is based on the understanding that at least the energy and spatial distortions of the camera can each be divided into two parts: a characteristic part and a specific part. The characteristic part does not vary significantly from camera to camera of a given design and/or does not change drastically with time for a given unit. The specific part may vary from camera to camera of a given design and/or with time for a particular camera. The characteristic part may be the result of the design or the construction of the particular camera. in either case the method contemplates the determination of either or both energy and dislocation corrections (and optionally the sensitivity correction) in two steps, a characteristic correction to correct for mainly intrinsic variations, including most of the errors and a specific correction to correct for errors, particular to a specific camera and/or to account for aging of the camera and/or a specific collimator and/or a specific energy range.
For a given detector configuration, the major portion of both energy and linearity distortion is usually caused by the common photomultiplier layout and other geometric attributes of the camera. A minor portion is related to specific system tolerances such as photomultiplier anisotropy, energy dependent average depth of photon interaction in the detector and other such causes. If the common major part is determined accurately, then the remaining camera specific part can be handled efficiently using lower count statistics and/or fewer images while providing the same or improved accuracy. This is true even if different cameras do not have the same exact geometric layout. in such a case, small geometrical differences between the layouts are corrected for as part of the minor correction. This method is most useful when the major portion of the distortion is common a group of cameras, however, this method also has utility when only a minor portion of the distortion is common to the cameras.
In one preferred embodiment of the invention, the sensitivity correction is divided into two (or three) parts. A first part is the portion of the correction which is specific to a particular collimator. This correction is determined for the collimator and is utilized by the camera whenever the particular collimator is used. Since this first part is camera independent, it does not need to be recalibrated for different cameras. The second portion of the correction is that caused by incomplete correction by the linearity and energy corrections, resulting in residual non-uniformity. Optionally, the sensitivity correction comprises a third portion which is caused by non-uniform sensitivity. of the scintillator crystal to incoming radiation. Alternatively, this portion may be included in the second portion. In a preferred embodiment of the invention, the first correction is determined separately of the camera on a per collimator basis. A flood image is first corrected for linearity and energy. The corrected flood is then used as the basis for a first sensitivity correction. The flood used for determining this correction can be acquired using a point source, giving a highly accurate correction. In use, this first correction is combined with the collimator correction to give an overall sensitivity correction. The advantage of this procedure is that a point source flood can be used to acquire the image needed for the first sensitivity correction, while a sheet source would be needed for determining the sensitivity correction, if the sensitivity is determined for the camera including the collimator. A sheet source flood is generally more difficult to perform and the radiation is generally less uniform,
In particular, in one preferred embodiment of the invention, the characteristic energy correction and/or the dislocation correction are determined for a xe2x80x9ctypicalxe2x80x9d camera of a given design and construction. This determination may be based on either or both a calculation of expected distortions and a very high precision, high count mapping of one or more cameras.
Alternatively, or additionally, characteristic energy and/or dislocation corrections are determined with great precision at the factory and optionally during installation or after a major repair of the camera
In either case, from time to time a second specific correction map (or maps) is determined based on the image(s) acquired as corrected by the characteristic maps. These specific correction maps make only relatively minor corrections in the images and hence need only be made using lesser statistics and/or lower spatial resolution than would be necessary in the absence of the characteristic maps. Optionally, when the amount of correction as determined for a specific correction map is greater than a given amount, the characteristic mapping process is carried out. The specific correction map is preferably determined using events as they are acquired. Alternatively, stored acquisition data may be used for off-line calibration.
Such specific correction maps may be acquired after minor repairs of the camera, such as replacing a PMT or with aging of the PMTs, which changes their operating characteristics.
In another aspect of the invention, changes in correction maps are adjusted with aging based on incremental corrections to previous maps. In this aspect of the invention, incremental corrections are determined based on old maps which are updated based on the determined incremental corrections. As a result, recalibrating the camera periodically to compensate for changing characteristics of the PMTs and the detector crystal is easier and/or faster.
In a preferred embodiment of the invention, calibration for a new collimator may be performed by first installing a supplied xe2x80x9cfirst stagexe2x80x9d calibration result and then proceeding directly to the second stage. Alternatively, as the collimator affects mostly the sensitivity errors, the additional calibration may be a third stage which is performed based on a supplied second stage.
Another aspect of the present invention relates to using a neural network (NN) instead of maps, to perform one or more steps of the above xe2x80x9ctriple stepxe2x80x9d correction method. A NN has the utility that it can be incrementally calibrated as described hereinabove.
A nuclear medicine camera in accordance with a preferred embodiment of the invention, uses a neural network (NN) for correcting linearity errors therein. Alternatively or additionally, the NN is also used for correcting energy errors. Additionally or alternatively, the NN is used to correct sensitivity errors.
In a preferred embodiment of the invention, a NN evaluates x, y and/or E and by-passes the Anger method. The NN is used to evaluate a function xe2x80x9cfxe2x80x9d which maps the PMT responses to the location (x, y) and the energy (E) of an event. Distortions and errors may be corrected by the NN or, alternatively, using maps, as known in the art.
In a preferred embodiment of the invention where the NN corrects linearity errors, the NN inputs preferably comprise an Anger calculated location (x, y) and the outputs of the NN preferably comprise a corrected location (x, y). During training, an additional input, a correct location (x, y) is preferably also used. Thus, the function f will typically be f(x, y)xe2x86x92(xxe2x80x2, yxe2x80x2).
In a preferred embodiment of the invention where the NN is used to perform the calculation of position and/or energy, the inputs preferably comprise the PMT data (integrated or raw) and the outputs preferably comprise a location (x, y) or a location and an energy (x, y, E). Thus, the function f may be f(PMT)xe2x86x92(x, y, E); f(PMT)xe2x86x92(x, y); f(PMT)xe2x86x92(E); or even f(PMT)xe2x86x92(x).
The NN is preferably operated in one of two modes, a learning mode, whereby the NN learns the characteristics of the camera and an output mode, whereby the NN generates corrected positions responsive to a PMT response set. The learning mode usually comprises two stages, a first, slow convergence stage where the NN learns the general characteristics of the function xe2x80x9cfxe2x80x9d and a second, fast conversion stage, where the NN fine-tunes to the particularities of the function xe2x80x9cfxe2x80x9d. It should be noted that a significant portion of the slow convergence stage has to do with idiosyncrasies of a typical NN learning process and not on the distribution of the distortions between the characteristic and specific types of distortions.
In preferred embodiments of the present invention, using a NN corresponds to using. incremental correction maps as described herein, In one such embodiment, the first learning stage corresponds to the generation of the characteristic correction and the second stage corresponds to the generation of the specific correction. Alternatively or additionally, applying an incremental correction is achieved by continuing to train a fully trained NN with new stimuli.
In a preferred embodiment of the invention, a NN comprises a plurality of independent NNs, each of which has a single output (one of x, y and E). These independent NNs can be operated in parallel for a higher throughput.
In accordance with another preferred embodiment of the invention, a NN is used to separate two temporally close events. The NN is trained with both single events and with combinations of events. These combinations are preferably computer generated by adding the PMT responses of two real events. Preferably, when adding the responses, a model of the PMT response is used to compensate for non-linearities in the response of the PMT. Preferably, this multiple-event stage of learning is applied after the second stage of learning (for single events). Alternatively, the second stage and the multiple-event stage of learning are combined. In a preferred embodiment of the invention, only one of the multiple events is detected and the other is ignored. Alternatively, two such temporally close events are separated and detected. This requires that the NN have more than one set of x and y (and E) outputs. In a preferred embodiment of the invention, the NN detects Compton scattering events which occur inside the crystal. Preferably, the location of the event is determined to be the location of the higher energy event of two events which are deemed to be a Compton scatter event.
In a preferred embodiment of the invention, a NN has one or more classification outputs, which define the type of event, such as which energy window it belongs to, whether it is temporally overlapping event, a spatially overlapping event, a Compton scatter event or a multiple Compton scatter event. The classification output may be of a binary type, i.e., each individual classification output indicates belonging to a single classification or it may be a multi-values output, each value corresponding to a classification.
While the invention is described in the context of corrections for linearity, energy and sensitivity, it is also applicable to other corrections which can be divided into a fairly constant major portion and a relatively small variable portion, where the variable portion may comprise, at least in part, variations which may occur in the major portion.
There is therefore provided, in accordance with a preferred embodiment of the invention, a method of correcting errors in imaging data in a Gamma Camera comprising:
determining a first correction map based on one or both of (1) calculated corrections and (2) a first data acquisition;
determining a second correction map based on a second data acquisition; and
correcting the imaging data based on the first and second correction maps.
In a preferred embodiment of the invention, the first correction map is based on a first data acquisition. This map may be based, in a preferred embodiment of the invention, on data acquisitions made on a typical camera or on one or a plurality of xe2x80x9cstandardxe2x80x9d gamma cameras. Alternatively or additionally, the first correction map comprises a combined correction map of at least two previous correction maps for a same type of error correction.
Alternatively or additionally, the first correction map and the second correction map are based on different photon energies. Thus, the first map can be acquired for a first photon energy and the second, fine tuning map, be acquired at the target photon energy. This method is also useful for converting a combined correction map for a first photon energy to a combined correction map for a second photon energy.
In a preferred embodiment of the invention, the first data acquisition is a relatively high count acquisition and the second acquisition is a relatively low count acquisition.
In a preferred embodiment of the invention, the first data acquisition is made at a relatively high spatial resolution and the second data acquisition is made at a relatively low spatial resolution.
In preferred embodiments of the invention, the first correction map is based on calculated corrections, or on both calculated corrections and a data acquisition.
In a preferred embodiment of the invention, the second correction map is determined periodically. Alternatively, or additionally, the second correction map is determined in response to perceived errors in an image produced by the camera. Alternatively or additionally, the second correction map is made after installation of the gamma camera.
In a preferred embodiment of the invention, the second correction map is determined without redetermining the first correction map.
In a preferred embodiment of the invention, the second correction map is combined with a prior correction map to determine a new correction map. In one preferred embodiment of the invention, the prior correction map is a prior second correction map and the new correction map is a new second correction map. Alternatively or additionally, the first and second correction maps are combined to form a combined correction map which is used to correct the acquired data
In a preferred embodiment of the invention, the imaging data is corrected by a plurality of corrections which correct for different errors in the data and wherein the first and second correction maps are determined for at least one of the corrections. Preferably, the at least one correction comprises an energy correction. Alternatively or additionally, the at least one correction comprises a dislocation correction. Preferably, the at least one correction comprises a sensitivity correction. Alternatively, the corrections comprise a sensitivity correction determined on the basis of a single correction map.
In an embodiment of the invention, the at least one correction comprises a flood correction.
There is further provided, in accordance with a preferred embodiment of the invention, a method for correcting an acquired image on a gamma camera comprising: determining a first correction map to correct for characteristic distortions; determining a second correction map to correct for specific distortions; and correcting the acquired image based on the first and the second correction maps.
There is also provided in accordance with a preferred embodiment of the invention a nuclear medicine camera, comprising:
a radiation detector which detects radiation events which impinge on the detector; and
a position and energy calculator which calculates at least one parameter of an interaction between an impinging radiation event and the detector,
wherein the calculator comprises a neural network.
Preferably, the parameter comprises a position coordinate of the event. Further preferably, the neural network calculates a second position coordinate of the event, to yield a two-dimensional position estimate of the event.
Alternatively or additionally, the parameter comprises a classification of the event. Alternatively or additionally, the parameter comprises an. association of the event with an energy window. Alternatively or additionally, the parameter comprises an energy of the event.
Alternatively or additionally, said parameter is calculated responsive to a state of the camera. Preferably, the state of the camera is a rotational state of the detector.
Alternatively or additionally, the NN receives inputs from the detector, which inputs are not integrated over an event duration. Alternatively, the NN receives inputs from the detector, which inputs are integrated over an event duration.
In a preferred embodiment of the invention, the detector comprises a solid state detector. Alternatively or additionally, the detector comprises a plurality of photomultipliers.
In a preferred embodiment of the invention, the parameter is calculated by said calculator without an interaction with the neural network. Preferably, said parameter comprises a position parameter, calculated using a center of gravity calculation. Alternatively or additionally, the neural network corrects a calculation of said parameter, which parameter is previously calculated without an interaction with the neural network.
In a preferred embodiment of the invention, the neural network separates two temporally overlapping events. In one embodiment of the invention, the neural network outputs a parameter of only a single one of said two events.
In a preferred embodiment of the invention, the events are generated by a radiation source which radiates at a plurality of radiation energies.
Alternatively or additionally, the neural network identifies Compton scatter events.
There is also provided for in a preferred embodiment of the invention, a method of calibrating a position and energy calculation unit of nuclear medicine camera to calculate a parameter of a radiation event, comprising:
exposing the calculator to a first plurality of radiation events; and
exposing the calculator to a second plurality of radiation events,
wherein, at least one of said first and said second pluralities of radiation events comprises simulated responses to radiation events.
Preferably, exposing to simulating events comprises irradiating a PMT portion of the camera with light flashes, which correspond to the simulated events.
There is provided in accordance with another preferred embodiment of the invention, a method of calibrating a position and energy calculation unit of nuclear medicine camera to calculate a parameter of a radiation event, comprising:
programming the calculator to a first level of precision, without exposing the calculator to radiation events; and
exposing the calculator to a plurality of radiation events.
Preferably, programming comprises providing an at least partially calibrated programming. Further preferably, providing an at least partially calibrated programming comprises copying said programming from a second nuclear medicine camera.
Alternatively or additionally, providing an at least partially calibrated programming comprises providing said programming from a different configuration of the nuclear medicine camera. Preferably, said different configuration comprises a configuration before replacing photomultiplier tubes. Alternatively or additionally, said different configuration comprises a configuration using a different collimator.
Alternatively or additionally, the method comprises simulated exposing of said calculator to a simulated plurality of overlapping radiation events after said exposing the calculator. Preferably, said simulated exposing comprises generating simulated overlapping events from individual events.
In a preferred embodiment of the invention, said calculator comprises a neural network.
There is also provided in accordance with a preferred embodiment of the invention, a method of calibrating a nuclear medicine camera, comprising:
providing a camera; and
periodically calibrating the camera, based on a previous calibrated state of the camera, which calibrating is exclusive of gain setting for individual PMTs of the camera.
Preferably, calibrating comprises calibrating a neural network portion of the camera. Alternatively or additionally, periodically calibrating comprises calibrating linearity corrections. Alternatively or additionally, periodically calibrating comprises calibrating energy corrections. Alternatively or additionally, periodically calibrating comprises calibrating more often than four times a year. Alternatively or additionally, periodically calibrating comprises calibrating at least once a month. Alternatively or additionally, periodically calibrating comprises calibrating at least once a week.
There is provided in accordance with another preferred embodiment of the invention, a collimator kit for a nuclear medicine camera, comprising:
a collimator; and
a programmed calibration for the combined camera and collimator.
There is further provided in accordance with a preferred embodiment of the invention, a collimator kit for a nuclear medicine camera, comprising:
a collimator; and
a programmed calibration for camera-independent sensitivity errors of the collimator.
Preferably, the kit comprises a programmed calibration for camera dependent sensitivity errors.